There are many research resource repositories here. However, be advised that the fact that something is listed in one of these places does NOT mean that it's reliable.
Kitty's Library is a large collection of links to online resources.
The "WikiTree Library" has an enormous collection of WikiTree pages about sources, almost all of which are free online books. Most of these pages simply document the availability and publication details of the book -- they don't tell you whether it's reliable. New pages can be created to add to the library.
Category:Reliable Sources for Pre-1700 Profiles contains guidance pages to assist in determining whether a source is reliable (or unreliable). It's organized by subject matter (topics and regions) related to WikiTree Projects.
Many Projects and sub-projects have pages and categories that cover the research resources for the project's subject matter.
And research resources are highly specific to the subject matter. Therefore, if you can't easily find what you are looking for, post a question here in G2G, and you are likely to get specific advice.
